PCBs are components of electronic devices which are made from laminates which comprise a conductive foil, usually copper, and a polymeric resin substrate. The conductive foil forms the conductors in electronic devices and the polymeric resin substrate forms an insulation between the conductors. The conductor and insulator are in intimate contact and the adhesion between them contributes to the performance and reliability of the electronic devices made with them.
Electrodeposited and wrought or rolled copper foils used in the manufacture of printed circuit boards do not adhere well to the polymeric substrates. The prior practice for achieving adhesion between copper foil and insulating polymeric substrates has been to roughen the copper surface.
Surface roughening has been achieved by several means. The electrodeposited copper foils can be electroformed with a rough surface. On top of this rough surface further toughening is carried out by applying a high surface area treatment. These treatments may be a copper deposited electrolytically in nodular or powder form, or a copper oxide which grows nodular or dendritic, among others. Often times the rolled copper foil has mechanical roughness imparted to it during rolling or by subsequent abrasion. The rolled foils also are conventionally treated with surface area increasing nodular copper or copper oxide treatments.
These surface roughening treatments increase adhesion to the polymers by forming a mechanical interlock with the resin. The mechanical interlock is formed when an adhesive in its liquid state is applied and then cured or when the resin melts and flows prior to cure during lamination. The polymers flow around the roughened surface area treatments to form the mechanical interlock.
There are several factors contributing to the adhesion measured between the copper foil and the polymeric resin. Some of these are surface area, type of roughness, wettability, chemical bond formation, type of chemical bond, formation of interpenetrating networks, and properties of the adhering materials.
During an adhesion test the interlocked resin and copper often adhere well enough that failure occurs within the resin, cohesive failure. With some resins the mechanical interlocking of treatment and resin does not result in the desired high adhesion and failure occurs at the interface between resin and copper, an adhesive failure.
The surface roughening that has been used to enhance adhesion between copper and polymeric resin substrates cause difficulties in the manufacture of PCBs and contributes to poor PCB performance. In the subtractive copper etching process additional etching time is required to remove the dendrites or nodules embedded in the resin. This not only slows down the production process but contributes to greater line loss due to the lateral etching of the copper line's sidewalls. The surface roughening contributes to poor PCB electrical performance by slowing down high frequency electrical signals. The necessity of having a rough base foil has limited other properties, such as tensile strength and elongation, that produce good laminate and PCB performance. The dendritic or nodular surface roughening treatments are difficult to apply, requiring special equipment in the case of electrolytic treatment, and special chemicals in the case of the oxide treatments.
The copper oxide process for increasing surface roughness is sometimes referred to as a brown or black oxide process. This oxide process includes a number of steps: (1) alkaline clean, (2) water rinse, (3) microetch, (4) acid rinse, (5) rinse, (6) oxidation, and (7) water rinse. This process provides adhesion or peel strength after lamination and post baking of only 2-4 pounds per inch for polyimide innerlayers and 4-6 pounds per inch for tetrafunctional epoxy innerlayers. Because of this limited adhesion, the interface between the oxide of the copper foil and the resin sometimes delaminates or fractures during the drilling process of making multilayer PCBs. The fractured interface near the edge of the drilled holes is prone to attack by acids during subsequent PCB processes. Acid sometimes leaches into the interface area around the drilled holes and dissolves the copper oxide resulting in a defect called pink ring or haloing around the drilled holes.
Double-treated copper foil is sometimes used as an alternative to the brown or black oxide process. Adhesion between the innerlayer, double treated copper foil and polyimide or epoxy resins is acceptable with the typical peel strength values of &gt;3 pounds per inch for polyimide resins and &gt;6 pounds per inch for epoxy resins. The interface between the double treated copper foil surface and these resins is stronger than that when brown or black oxide is used and is, therefore, very resistent to fracturing delamination, and acid attack or pink ring defects. However, double treated copper foil has not gained wide acceptance by multilayer PCB manufacturers because of its high cost.
Japanese Patent Application Publication (Kokoku) No. 60-15654 discloses a method for adhesively attaching a chrome-treated layer of copper foil to a resin substrate. In a first embodiment the chrome-treated layer is formed on the rough or matte side of the copper foil. This chrome-treated layer is treated with a solution of a silane coupling agent represented by the formula YRSiX.sub.3, wherein Y is a functional group that is reactive with a high polymer, R is a bonding group which includes a chain-like or a cyclic hydrocarbon connecting Y and Si, and X is a hydrolyzable organic or inorganic group. In a second embodiment the chrome-treated layer is first formed on the rough side of the copper foil and is then impregnated with a silane coupling agent represented by the above formula. With each embodiment the treated surface is bonded to the resin substrate.
Japanese Patent Application Publication (Kokoku) No. 2-19994 discloses copper foil for use in making printed circuits wherein a layer of zinc or zinc alloy is formed on the rough or matte side of the copper foil. A chrome-treated layer is formed on the zinc or zinc alloy layer. This chrome-treated layer is treated with a silane coupling agent represented by the formula YRSiX.sub.3, wherein Y is a functional group that is reactive with a high polymer, R is a bonding group which includes a chain-like or a cyclic hydrocarbon connecting Y and Si, and X is a hydrolyzable organic or inorganic group.
U.S. Pat. No. 5,071,520 discloses a technique for improving the peel strength of wrought or electrolytically deposited copper foil having an anti-tarnish treatment applied thereto. The anti-tarnish treatment is applied using chromium or the combination of chromium and zinc. The treated foil is then subjected to a rinse containing a silane coupling agent.
U.S. Pat. No. 3,644,166 discloses a copper-clad glass epoxy laminate made with a treated copper foil sheet having an extremely thin oxidation-resistant amino-silane film applied to it. The copper foil is treated with chemicals on the bonding side to improve adhesion resulting in copper oxides being incorporated and made a part of the grain structure of the copper foil surface. The amino-silane film coats the toughened copper surface to prevent oxide transfer to the organic support and oxidation of the copper foil. The reference states that the amino-silane film can be applied to nontreated copper foil and to any other conductor metals if oxidation problems exist with such metals.